The present invention relates to a device for forming a thin film of SnO.sub.2, In.sub.2 O.sub.3, TiO.sub.2, SiO.sub.2, or the like on a surface of a substrate by spraying an atomized solution of the material over the surface of the substrate which is heated.
One known thin-film forming device of the type described is shown in FIG. 3 of the accompanying drawings. A substrate 2 to be coated with a thin film on a surface thereof is placed in a reaction chamber 1 with the substrate surface directed downwardly. The substrate 2 is heated by a heater 3 located behind the substrate 2. An atomizer 4 and an air blower 5 coupled therewith are disposed below the reaction chamber 1. A nozzle 6 connected to the atomizer 4 is supported in the reaction chamber 1 and opens toward the substrate 2.
An aqueous solution of a chloride such as SnCl.sub.4, InCl.sub.3, or the like is atomized in the atomizer 4, and the atomized solution is progressively sprayed from the nozzle 6 over the surface of the substrate 2 by the air delivered by a fan 9 in the air blower 5. The surface of the substrate 2 is heated to a temperature higher than the reaction temperature of the material in the solution. For example, the surface of the substrate 2 is heated to a temperature range of from 400.degree. to 500.degree. C. when forming a thin film of SnO.sub.2 on the substrate surface.
The atomized solution sprayed toward the surface of the substrate 2 is partially heated in the vicinity of the substrate surface, and dewatered and vaporized. The vaporized material then reacts with oxygen and water vapor in the air to form a thin film of an oxide of Sn, In, or the like on the surface of the substrate 2.
Most of the atomized solution which has been ejected from the nozzle 6 and is not responsible for forming the thin film on the substrate 2 is discharged in the atomized condition from an exhaust outlet 7. When the inner wall surfaces of the reaction chamber 1 are heated by radiation and convection from the heater 3, part of the atomized solution flowing along the inner wall surfaces of the reaction chamber 1 is also dewatered and vaporized. This vaporized material also reacts with oxygen and water vapor and is deposited as a coating on the inner wall surfaces of the reaction chamber 1.
The coating thus deposited on the inner wall surface of the reaction chamber 1 grows as thin films are formed on successive substrates 2. When the deposited coating grows to the thickness of about 4 micrometers, then it is liable to crack due to internal strains, and broken into small fragments which come off the inner wall surfaces and are scattered into the reaction chamber 1. Some of the scattered pieces are applied to the surface of the substrate 2 to develop unwanted defects such as pinholes in the thin film formed on the surface of the substrate 2.